The present invention comprises an optical train (50) and optional wavelength-selective photodetectors. The optical train (50) uses reflecting elements (600) including mirrors and/or prisms to fold the light path of the transmitted UV light beam to direct it through the body (100) of the instrument, through a sample vessel (200) using at least one pass but preferably two or more passes and into illumination contact with a photodetector (400). With each additional pass, the Beer-Lambert path length is effectively increased. Separate second optical train (53) and third optical train (54) exist for the detection and measurement of scattered light by illumination contact with one or more photodetectors.